Methods and systems for motion vector derivation at a video decoder

ABSTRACT

Method and apparatus for deriving a motion vector at a video decoder. A block-based motion vector may be produced at the video decoder by utilizing motion estimation among available pixels relative to blocks in one or more reference frames. The available pixels could be, for example, spatially neighboring blocks in the sequential scan coding order of a current frame, blocks in a previously decoded frame, or blocks in a downsampled frame in a lower pyramid when layered coding has been used.

CLAIM FOR PRIORITY

This application is a continuation of U.S. patent application Ser. No.16/552,995, filed on 27 Aug. 2019, now U.S. Pat. No. 10,863,194,entitled “METHODS AND SYSTEMS FOR MOTION VECTOR DERIVATION”, which is acontinuation of U.S. patent application Ser. No. 15/960,120, filed on 23Apr. 2018, now U.S. Pat. No. 10,404,994, entitled “METHODS AND SYSTEMSFOR MOTION VECTOR DERIVATION”, which is a continuation of U.S. patentapplication Ser. No. 14/737,437, filed on 11 Jun. 2015, now U.S. Pat.No. 9,955,179, entitled “METHODS AND SYSTEMS FOR MOTION VECTORDERIVATION AT A VIDEO DECODER”, which is a continuation of U.S. patentapplication Ser. No. 12/567,540, filed on 25 Sep. 2009, now U.S. Pat.No. 9,654,792, entitled “METHODS AND SYSTEMS FOR MOTION VECTORDERIVATION AT A VIDEO DECODER”, which is a Non-Provisional applicationof U.S. Provisional Patent Application Ser. No. 61/222,984, filed on 3Jul. 2009, entitled “METHODS AND SYSTEMS FOR MOTION VECTOR DERIVATION ATA VIDEO DECODER”, all of which are incorporated by reference in theirentireties for all purposes.

BACKGROUND

Motion estimation (ME) in video coding may be used to improve videocompression performance by removing or reducing temporal redundancyamong video frames. For encoding an input block, traditional motionestimation may be performed at an encoder within a specified searchwindow in reference frames. This may allow determination of a motionvector that meets a predefined requirement, such as the minimization ofa metric such as the sum of absolute differences (SAD) between the inputblock and the reference block. The motion vector (MV) information canthen be transmitted to a decoder for motion compensation. The videodecoder may then utilize the received motion vector information todisplace the pixels from the reference frames to form reconstructedoutput pixels. This displacement may be used to represent the motioncompensation.

Note that in the description below, the terms “frame” and “picture” areused interchangeably, as would be understood by persons of ordinaryskill in the art.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

FIG. 1 illustrates MV derivation using already decoded blocks from acurrent frame, according to an embodiment.

FIG. 2 is a flow chart illustrating the process of MV derivation usingalready decoded blocks from a current frame, according to an embodiment.

FIG. 3 illustrates MV derivation using already decoded blocks from acurrent frame, where the motion search is based on blocks in twodifferent reference frames, according to an embodiment.

FIG. 4 is a flow chart illustrating MV derivation using already decodedblocks from a current frame, where the motion search is based on blocksin two different reference frames, according to an embodiment.

FIG. 5 illustrates MV derivation using previously decoded blocks from apreviously decoded frame, according to an embodiment.

FIG. 6 is a flow chart illustrating MV derivation using previouslydecoded blocks from a previously decoded frame, according to anembodiment.

FIG. 7 illustrates MV derivation using previously decoded blocks fromalready decoded previous and succeeding frames, according to anembodiment.

FIG. 8 is a flow chart illustrating MV derivation using previouslydecoded blocks from already decoded previous and succeeding frames,according to an embodiment.

FIG. 9 illustrates MV derivation using a previously decoded block from alower level in a layered coding context, according to an embodiment.

FIG. 10 is a flow chart illustrating MV derivation using a previouslydecoded block from a lower level in a layered coding context, accordingto an embodiment.

FIG. 11 illustrates a computing context of an exemplary softwareembodiment.

FIG. 12 is a block diagram showing a self MV derivation module in thecontext of a H.264 encoder, according to an embodiment.

FIG. 13 is a block diagram showing a self MV derivation module in thecontext of a H.264 decoder, according to an embodiment.

FIG. 14 is a block diagram illustrating a system, according to anembodiment.

DETAILED DESCRIPTION

The following applies to video compression. The system and methoddescribed below may allow derivation of a motion vector (MV) at a videodecoder. This can reduce the amount of information that needs to be sentfrom a video encoder to the video decoder. A block-based motion vectormay be produced at the video decoder by performing motion estimation onavailable previously decoded pixels with respect to blocks in one ormore reference frames. The available pixels could be, for example,spatially neighboring blocks in the sequential scan coding order of thecurrent frame, blocks in a previously decoded frame, or blocks in adownsampled frame in a lower pyramid when layered coding has been used.In an alternative embodiment, the available pixels can be a combinationof the above-mentioned blocks.

Already Decoded Spatially Neighboring Blocks in the Current Frame

In an embodiment, pixels that can be used to determine an MV may comefrom spatially neighboring blocks in the current frame, where theseblocks have been decoded prior to the decoding of the target block inthe current frame. FIG. 1 shows an example 100 utilizing one or moreblocks 140 that are above and to the left of the target block 130 in acurrent frame 110. To determine a motion vector for the target block 130that needs to be decoded in the current frame 110, motion search may beperformed for one or more of the blocks 140 above and to the left of thetarget block 130, relative to the blocks 150 of reference frame 120,where blocks 150 correspond to blocks 140. Such an approach may beuseful in decoding of predictive frames, also called P-frames, whichhold only the changes relative to a previous frame.

In an embodiment, the raster scan coding order may be used to identifythe spatial neighbor blocks that are above, to the left, above and tothe left, and above and to the right of the target block.

Generally, this approach may be applied to available pixels of spatiallyneighboring blocks in the current frame, as long as the neighboringblocks were decoded prior to the target block in sequential scan codingorder. Moreover, this approach may apply motion search with respect toreference frames in the reference frame list for a current frame.

The processing for this embodiment is illustrated as process 200 in FIG.2 . At 210, one or more blocks of pixels may be identified, where theseblocks neighbor the target block of the current frame. Such neighboringblocks may or may not be immediately adjacent to the target block. At220, motion search may be performed for the identified blocks. Themotion search may be based on corresponding blocks of a reference frame,and at 230 yields motion vectors that connects the corresponding blocksof the reference frame to the identified blocks. Note that in anembodiment, 220 and 230 may precede 210, such that the motion vectorsfor the identified blocks may be known and the identified blocksdecoded, prior to the identified blocks being used in this process. At240, the motion vectors of the identified blocks are used to derive themotion vector for the target block, which may then be used for motioncompensation for the target block. This derivation may be performedusing any suitable process known to persons of ordinary skill in theart. Such a process may be, for example and without limitation, weightedaveraging or median filtering. The process 200 concludes at 250.

FIG. 3 shows an embodiment 300 that may utilize one or more neighboringblocks 340 (shown here as blocks above and to the left of the targetblock 330) in a current frame 310. This may allow generation of a motionvector based on one or more corresponding blocks 350 and 355 in aprevious reference frame 320 and a subsequent reference frame 360,respectively, where the terms “previous” and “subsequent” refer totemporal order. The motion vector can then be applied to target block330. Here, the motion search may operate over an additional referenceframe, in contrast to the embodiments of FIGS. 1 and 2 . In anembodiment, a raster scan coding order may be used to determine spatialneighbor blocks above, to the left, above and to the left, and above andto the right of the target block. This approach may be used forbi-directional (B) frames, which use both the preceding and followingframes for decoding.

The approach exemplified by FIG. 3 may be applied to available pixels ofspatially neighboring blocks in a current frame, as long as theneighboring blocks were decoded prior to the target block in sequentialscan coding order. Moreover, this approach may apply motion search withrespect to reference frames in reference frame lists for a currentframe.

The process of the embodiment of FIG. 3 is shown as process 400 of FIG.4 . At 410, one or more blocks of pixels may be identified in thecurrent frame, where the identified blocks neighbor the target block ofthe current frame. At 420, motion search for the identified blocks maybe performed, based on corresponding blocks in a temporally subsequentreference frame and on corresponding blocks in a previous referenceframe. At 430, the motion search may result in motion vectors for theidentified blocks. As in the case of FIGS. 2, 420 and 430 may precede410, such that the motion vectors of the neighboring blocks may bedetermined prior to identification of those blocks. At 440, the motionvectors may be used to derive the motion vector for the target block,which may then be used for motion compensation for the target block.This derivation may be performed using any suitable process known topersons of ordinary skill in the art. Such a process may be, for exampleand without limitation, weighted averaging or median filtering. Theprocess concludes at 450.

Already Decoded Blocks in Previously Decoded Frames

In this embodiment, pixels that can be used to determine an MV may comefrom a corresponding block in a previously reconstructed frame. FIG. 5shows an example 500 of utilizing a block 540 from a previous frame 515,where the block 540 may be in a position corresponding to a target block530 in a current frame 510. Here the MV can be derived from therelationship between the corresponding block 540 of the previouslydecoded frame 515 relative to one or more blocks 550 in a referenceframe 520.

The processing for such an embodiment is shown in FIG. 6 as process 600.At 610, a block of pixels may be identified in a previous frame, wherethe identified block corresponds to a target block of a current frame.At 620, a motion vector may be determined for the identified blockrelative to a corresponding block in a reference frame. In analternative embodiment, 620 may precede 610, such that the motion vectorfor the block of the previous frame may be derived prior to identifyingthe block for use with respect to the target block of the current frame.At 630, the motion vector may be used for the target block. The processmay conclude at 640.

Another embodiment may use neighboring blocks next to the correspondingblock of the previous frame to do the motion search in a referenceframe. Examples of such neighboring blocks could be the blocks above,below, to the left, or to the right of the corresponding block in thepreviously reconstructed frame.

In an alternative embodiment, the available pixels can come from thecorresponding blocks of previous and succeeding reconstructed frames intemporal order. This approach is illustrated in FIG. 7 as embodiment700. To encode a target block 730 in a current frame 710, alreadydecoded pixels may be used, where these pixels may be found in acorresponding block 740 of a previous frame 715, and in a correspondingblock 765 of a succeeding frame 755. A first motion vector may bederived for corresponding block 740, by doing a motion search throughone or more blocks 750 of reference frame 720. Block(s) 750 may neighbora block in reference frame 720 that corresponds to block 740 of previousframe 715. A second motion vector may be derived for corresponding block765 of succeeding frame 755, by doing a motion search through one ormore blocks 770 of reference frame 760. Block(s) 770 may neighbor ablock in reference frame 760 that corresponds to block 765 of succeedingframe 755. Based on the first and second motion vectors, forward and/orbackward motion vectors for target block 730 may be determined. Theselatter motion vectors may then be used for motion compensation for thetarget block

This process is described as process 800 of FIG. 8 . At 810, a block maybe identified in a previous frame, where this identified block maycorrespond to the target block of the current frame. At 820, a firstmotion vector may be determined for this identified block of theprevious frame, where the first motion vector may be defined relative toa corresponding block of a first reference frame. In 830, a block may beidentified in a succeeding frame, where this block may correspond to thetarget block of the current frame. A second motion vector may bedetermined at 840 for this identified block of the succeeding frame,where the second motion vector may be defined relative to thecorresponding block of a second reference frame. At 850, one or twomotion vectors may be determined for the target block using therespective first and second motion vectors above. Process 800 mayconclude at 860.

In another embodiment, neighboring blocks next to the correspondingblock in the previous and succeeding reconstructed frames may be used todo the motion search based on their respective reference frames. Anexample of the neighboring blocks may be the blocks above, below, to theleft, or to the right of the collocated blocks in the reconstructedframes, for example. Moreover, this approach can use motion search usingreference frames in the reference frame lists, in both forward andbackward temporal order.

Generally, the approach of FIGS. 3 and 7 may be used in the codecprocessing of bi-directional (B) frames.

Already Decoded Blocks in a Downsampled Frame in a Lower Pyramid ofLayered Coding

In an embodiment, pixels that can be used to determine an MV for atarget block may come from corresponding blocks in a lower layer whosevideo is downsampled from an original input in a scalable video codingscenario. FIG. 9 shows an example 900 utilizing a lower layer block 940corresponding to the target block 930 of the current picture 910. Theblock 940 may occur in a picture 915 that corresponds to current picture910. The corresponding block 940 can be used to perform the motionsearch, given one or more blocks 950 and 970 in respective referencepictures 920 and 960 in the lower layer. The reference pictures in thelower layer can be the forward or backward (previous or succeeding)pictures in temporal order. Since the motion vector may be derived inthe downsampled layer, the motion vector may be upscaled before it isapplied to the target block 930 in the target layer.

This approach may also be applied to already-decoded blocks that arespatial neighbors to the block 940 in the lower layer corresponding tothe target frame 930 in the current picture 910.

The processing of FIG. 9 is shown as a flowchart 1000 in FIG. 10 . At1010, given a target block in a current frame, a corresponding block maybe identified in a corresponding frame in a lower layer. At 1020, amotion vector may be determined for the corresponding block in the lowerlayer, relative to one or more reference frames in the lower layer. At1030, the determined motion vector may be used for motion estimation forthe target block in the current frame. The process may conclude at 1040.

In an alternative embodiment, 1020 may precede 1010, so that the motionvector is determined at the lower layer, prior to identifying the blockin the lower layer for ME purposes for the target layer.

Mode Selection

A rate distortion optimization (RDO) model may be used to determinewhich coding mode is selected, given the options of motion estimation atvideo encoder side and motion estimation at video decoder side. The RDOmodel for motion estimation at the video encoder may generate a costmetric, and may include the costs of both coding distortion and MV bits,and the cost function for the motion estimation at the decoder mayinclude only the coding distortion. In an embodiment, the video encodermay compare the costs for these two motion estimation options anddetermine which one to pick. In an embodiment, the video encoder mayidentify the chosen coding mode with a flag bit during communicationsbetween the encoder and the decoder. The video decoder may then actaccording to the state of the flag bit. If the flag bit indicates thatmotion estimation at the decoder side is utilized, the video decoder mayderive the motion vector autonomously.

Such a mode selection process is illustrated in FIG. 11 , as process1100. At 1120, traditional encoder side motion estimation (ME) may firstbe performed to get an MV for this coding mode. At 1130, thecorresponding RDO cost metric may be calculated. Let this cost be J0. At1140, ME is performed at the decoder as described in any of the aboveembodiments, to get an MV for this coding mode. At 1150, thecorresponding RDO cost metric may be calculated to be J1. At 1160, ifJ1<J0, then at 1170, the decoder side ME based result may be chosen.Otherwise, the result from the traditional ME based coding mode may bechosen at 1180. The process may conclude at 1190. In an alternativeembodiment, more than two modes may be similarly evaluated, where themode having the lowest RDO cost metric may be chosen. A flag can be usedto signal the chosen mode in the communications between the encoder anddecoder.

System

Logic to perform the processing described above may be incorporated in aself MV derivation module that is used in a larger codec architecture.FIG. 12 illustrates an exemplary H.264 video encoder architecture 1200that may include a self MV derivation module 1240, where H.264 is avideo codec standard. Current video information may be provided from acurrent video block 1210 in a form of a plurality of frames. The currentvideo may be passed to a differencing unit 1211. The differencing unit1211 may be part of the Differential Pulse Code Modulation (DPCM) (alsocalled the core video encoding) loop, which may include a motioncompensation stage 1222 and a motion estimation stage 1218. The loop mayalso include an intra prediction stage 1220, and intra interpolationstage 1224. In some cases, an in-loop deblocking filter 1226 may also beused in the loop.

The current video may be provided to the differencing unit 1211 and tothe motion estimation stage 1218. The motion compensation stage 1222 orthe intra interpolation stage 1224 may produce an output through aswitch 1223 that may then be subtracted from the current video 1210 toproduce a residual. The residual may then be transformed and quantizedat transform/quantization stage 1212 and subjected to entropy encodingin block 1214. A channel output results at block 1216.

The output of motion compensation stage 1222 or inter-interpolationstage 1224 may be provided to a summer 1233 that may also receive aninput from inverse quantization unit 1230 and inverse transform unit1232. These latter two units may undo the transformation andquantization of the transform/quantization stage 1212. The inversetransform unit 1232 may provide dequantized and detransformedinformation back to the loop.

A self MV derivation module 1240 may implement the processing describedherein for derivation of a motion vector from previously decoded pixels.Self MV derivation module 1240 may receive the output of in-loopdeblocking filter 1226, and may provide an output to motion compensationstage 1222.

FIG. 13 illustrates an H.264 video decoder 1300 with a self MVderivation module 1310. Here, a decoder 1300 for the encoder 1200 ofFIG. 12 may include a channel input 1338 coupled to an entropy decodingunit 1340. The output from the decoding unit 1340 may be provided to aninverse quantization unit 1342 and an inverse transform unit 1344, andto self MV derivation module 1310. The self MV derivation module 1310may be coupled to a motion compensation unit 1348. The output of theentropy decoding unit 1340 may also be provided to intra interpolationunit 1354, which may feed a selector switch 1323. The information fromthe inverse transform unit 1344, and either the motion compensation unit1348 or the intra interpolation unit 1354 as selected by the switch1323, may then be summed and provided to an in-loop de-blocking unit1346 and fed back to intra interpolation unit 1354. The output of thein-loop deblocking unit 1346 may then be fed to the self MV derivationmodule 1310.

The self MV derivation module may be located at the video encoder, andsynchronize with the video decoder side. The self MV derivation modulecould alternatively be applied on a generic video codec architecture,and is not limited to the H.264 coding architecture.

The encoder and decoder described above, and the processing performed bythem as described above, may be implemented in hardware, firmware, orsoftware, or some combination thereof. In addition, any one or morefeatures disclosed herein may be implemented in hardware, software,firmware, and combinations thereof, including discrete and integratedcircuit logic, application specific integrated circuit (ASIC) logic, andmicrocontrollers, and may be implemented as part of a domain-specificintegrated circuit package, or a combination of integrated circuitpackages. The term software, as used herein, refers to a computerprogram product including a computer readable medium having computerprogram logic stored therein to cause a computer system to perform oneor more features and/or combinations of features disclosed herein.

A software or firmware embodiment of the processing described above isillustrated in FIG. 14 . System 1400 may include a processor 1460 and abody of memory 1410 that may include one or more computer readable mediathat store computer program logic 1420. Memory 1410 may be implementedas a hard disk and drive, a removable media such as a compact disk anddrive, or a read-only memory (ROM) device, for example. Processor 1460and memory 1410 may be in communication using any of severaltechnologies known to one of ordinary skill in the art, such as a bus.Logic contained in memory 1410 may be read and executed by processor1460. One or more I/O ports and/or I/O devices, shown as I/O 1470, mayalso be connected to processor 1460 and memory 1410.

Computer program logic 1420 may include decoded block identificationlogic 1430. This module of computer program logic, when executed onprocessor 1460, identifies a block of pixels that may ultimately be usedto determine a motion vector for a target block. Computer program logic1420 may also include motion vector determination logic 1440. Thismodule of computer program logic, when executed on processor 1460,determines a motion vector on the basis of the identified block ofpixels identified by decoded block identification logic 1430, relativeto one or more reference frames. Computer program logic 1420 may alsoinclude motion vector application logic 1450. This module of computerprogram logic, when executed on processor 1460, uses the motion vectordetermined by logic module 1440 to perform motion estimation for thetarget block.

Alternatively, any of the logic modules shown in computer program logic1420 may be implemented in hardware.

Methods and systems are disclosed herein with the aid of functionalbuilding blocks, such as those listed above, describing the functions,features, and relationships thereof. At least some of the boundaries ofthese functional building blocks have been arbitrarily defined hereinfor the convenience of the description. Alternate boundaries may bedefined so long as the specified functions and relationships thereof areappropriately performed. In addition, the encoder and decoder describedabove may by incorporated in respective systems that encode a videosignal and decode the resulting encoded signal respectively using theprocesses noted above.

While various embodiments are disclosed herein, it should be understoodthat they have been presented by way of example only, and notlimitation. It will be apparent to persons skilled in the relevant artthat various changes in form and detail may be made therein withoutdeparting from the spirit and scope of the methods and systems disclosedherein. Thus, the breadth and scope of the claims should not be limitedby any of the exemplary embodiments disclosed herein.

What is claimed is:
 1. A video encoding method comprising: determining afirst cost metric corresponding to motion estimation at a video decoder,the first cost metric consisting of only coding distortion for motionestimation at the video decoder; determining a second cost metriccorresponding to motion estimation at a video encoder, the second costmetric comprising costs of coding distortion and motion vector bits formotion estimation at the video encoder; selecting a coding mode frommotion estimation at the video decoder and motion estimation at thevideo encoder based on a comparison of the first and second costmetrics; and identifying the selected coding mode for use in videodecode, wherein the determining the first cost metric comprisesperforming decoder side motion estimation and selecting a first motionvector for a first coding mode corresponding to the first cost metric,and the decoder side motion estimation comprises: selecting a firstblock of decoded pixels of a first reference frame, the first blockcorresponding to a first target block in a current frame; andidentifying the first motion vector for the first target block based onmotion estimation of a second reference frame using the first block. 2.The video encoding method of claim 1, wherein the first block is in aposition in the first reference frame corresponding to a position of thefirst target block in the current frame, the first reference frame istemporally previous to the current frame and the second reference frameis temporally previous to the first reference frame.
 3. The videoencoding method of claim 1, wherein the first cost metric is based onmotion compensation of the first target block using the first motionvector.
 4. The video encoding method of claim 1, further comprising:selecting a second block of decoded pixels of a third reference frame,the second block corresponding to the first target block, wherein thefirst motion vector is identified further based on motion estimation ofa fourth reference frame using the second block.
 5. A video encodercomprising: a memory; and one or more processors in communication withthe memory, the one or more processors being configured to: determine afirst cost metric corresponding to motion estimation at a video decoder,the first cost metric consisting of only coding distortion for motionestimation at the video decoder; determine a second cost metriccorresponding to motion estimation at a video encoder, the second costmetric comprising costs of coding distortion and motion vector bits formotion estimation at the video encoder; select a coding mode from motionestimation at the video decoder and motion estimation at the videoencoder based on a comparison of the first and second cost metrics; andidentify the selected coding mode for use in video decode, wherein todetermine the first cost metric, the one or more processors are furtherconfigured to perform decoder side motion estimation and select a firstmotion vector for a first coding mode corresponding to the first costmetric, and to perform the decoder side motion estimation, the one ormore processors are further configured to: select a first block ofdecoded pixels of a first reference frame, the first block correspondingto a first target block in a current frame; and identify the firstmotion vector for the first target block based on motion estimation of asecond reference frame using the first block.
 6. The video encoder ofclaim 5, wherein the first block is in a position in the first referenceframe corresponding to a position of the first target block in thecurrent frame, the first reference frame is temporally previous to thecurrent frame and the second reference frame is temporally previous tothe first reference frame.
 7. The video encoder of claim 5, wherein thefirst cost metric is based on motion compensation of the first targetblock using the first motion vector.
 8. The video encoder of claim 5,the one or more processors being further configured to: select a secondblock of decoded pixels of a third reference frame, the second blockcorresponding to the first target block, wherein the first motion vectoris identified further based on motion estimation of a fourth referenceframe using the second block.
 9. At least one non-transitorycomputer-readable medium comprising instructions stored thereon, whichif executed by one or more processors, cause the one or more processorsperform video encoding by: determining a first cost metric correspondingto motion estimation at a video decoder, the first cost metricconsisting of only coding distortion for motion estimation at the videodecoder; determining a second cost metric corresponding to motionestimation at a video encoder, the second cost metric comprising costsof coding distortion and motion vector bits for motion estimation at thevideo encoder; selecting a coding mode from motion estimation at thevideo decoder and motion estimation at the video encoder based on acomparison of the first and second cost metrics; and identifying theselected coding mode for use in video decode, wherein the determiningthe first cost metric comprises performing decoder side motionestimation and selecting a first motion vector for a first coding modecorresponding to the first cost metric, and the decoder side motionestimation comprises: selecting a first block of decoded pixels of afirst reference frame, the first block corresponding to a first targetblock in a current frame; and identifying the first motion vector forthe first target block based on motion estimation of a second referenceframe using the first block.
 10. The non-transitory computer-readablemedium of claim 9, wherein the first block is in a position in the firstreference frame corresponding to a position of the first target block inthe current frame, the first reference frame is temporally previous tothe current frame and the second reference frame is temporally previousto the first reference frame.
 11. The non-transitory computer-readablemedium of claim 9, wherein the first cost metric is based on motioncompensation of the first target block using the first motion vector.12. The non-transitory computer-readable medium of claim 9, wherein theinstructions, if executed by the one or more processors, further causethe one or more processors to: select a second block of decoded pixelsof a third reference frame, the second block corresponding to the firsttarget block, wherein the first motion vector is identified furtherbased on motion estimation of a fourth reference frame using the secondblock.